Well, obviously i'm a little biased being a big fan of vehicle combat in BF, but i feel that vehicles should be really strong. Not unkillable, mind you, but very strong and hard to deal with.
Here's why,
Infantry can spawn almost anywhere, nearly instantly. There is very little penalty and no real downtime when you die. You can be killed by a tank, and reappear seconds later in the same area and keep right on attacking it.
Vehicles, on the other hand have long CDs, and even if you wait for the CD, there's no guarantee you'll actually get it. And even if you do get it, most times you now have a long drive across half the map to get back to the action. There is a VERY limited amount of them and they are usually hard to get.
Now compare those two.
If armor dies easily, and is no more lethal than a guy on foot, why would anyone bother to go through all the drawbacks i just listed?
Armor NEEDS to be borderline OP, or there's no reason to use it.